As a means of printing an image (including text and symbols) on a printing medium such as paper or plastic film (for an OHP, for example) from input image information, an inkjet printing apparatus that is either built into or installed in a printer, facsimile machine, copier or the like is widely used in the prior art.
An inkjet printing apparatus prints by discharging ink droplets onto the printing medium from the printhead. Such apparatuses are easy to make compact, can print accurately at high speed, impose low running costs and are relatively quiet because they use a non-impact type of printing method. In addition, such apparatuses have the advantage of making color printing easy using multiple color ink.
These inkjet printing apparatuses are equipped with drive sources such as a carriage motor for reciprocally driving a carriage back and forth (hereinafter reciprocally driving) on which the printhead is mounted, an ASF (automatic sheet feeder) motor used for feeding a printing medium, a recovery system motor for head cleaning and the like, and a conveyance motor for conveying the printing medium with every printing scan of the printhead. Conventionally, stepping motors have been used in these types of drive sources because cost reductions come easily and control is simple.
In principle, an inkjet printing apparatus like that described above is relatively quiet because it uses a non-impact type printing method. However, it is becoming more common to use a DC motor as the above-described drive source in order to make the apparatus even quieter. In such cases, an encoder is typically used in order to obtain DC motor control information.
FIG. 10 is a schematic diagram showing an encoder structure.
The encoder, as shown in FIG. 10, is constructed so that a detector 703 detects light generated from an LED 701 via a code wheel 702 and generates a signal. On the code wheel 702 itself, alternating open portions through which light passes (704) and solid portions through which light does not pass (705) are disposed at set intervals, while photodiodes 706, 707, 708 and 709 are arranged at set intervals on the detector 703, with the light detected at each of the photodiodes 706-709 converted to electrical signal (A) 710, electrical signal (*A) 711, electrical signal (B) 712 and electrical signal (*B) 713, respectively, output, and the electrical signals 710-713 thus output are output by comparators 714 and 715 as differential output signals (channel A, channel B) 716, 717.
FIG. 11 is a signal waveform diagram showing a differential output signal waveform.
As shown in the diagram, a signal that inverts at the intersection of electrical signal (A) 801 and electrical signal (*A) 802 becomes channel A 803. If the carriage velocity is constant, ideally, the channel A 803 duty is 50 percent, that is, for one cycle of that signal, the time during which the signal level is HIGH and the time during which the signal level is LOW are identical (in FIG. 11, 50 percent each).
Generally, a signal that has been put through a digital LPF (low-pass filter) is used in order to eliminate noise when using a digital encoder signal.
FIG. 12 is a block diagram showing the structure of an LPF circuit.
As shown in FIG. 12, the LPF circuit forms a shift resister by connecting serially a plurality of DFF (D-type flip-flop). A digital encoder signal 605 is input to the shift resister and, each time a clock signal CLK 606 is input, sequentially the state of the DFF 601 is conveyed to DFF 602, the state of the DFF 602 is conveyed to DFF 603 and the state of DFF 603 is conveyed to DFF 604.
The Q outputs of each of the DFFs 602-604 are input to an AND circuit 607, and the output from the AND circuit 607 is connected to the J-input of a JKFF (J-K type flip-flop). At the same time, the inverted outputs of the DFFs 602-604 are input to another AND circuit 609 and the output of the AND circuit 609 is connected to the K-input of the JKFF 608.
By so doing, when all the output levels of the three DFFs 602-604 are HIGH, a HIGH signal is output from the AND circuit 607 and as a result the JKFF 608 outputs a HIGH signal. When all the output levels of the three DFFs 602-604 are low, a LOW signal is output from the AND circuit 607 and as a result the JKFF 608 outputs a LOW signal.
In short, only when the outputs of all three of the DFFs 602-604 match does the JKFF signal output level change. Accordingly, with a circuit of the structure shown in FIG. 12, in order to make the output from all three of the DFFs 602-604 match, the level of the digital encoder signal 605 must be constant for at least three clock signals or more.
In other words, signal changes that are shorter than 3 clock signal lengths in the digital encoder signal 605 are ignored.
In a structure of this type, when setting the LPF cut frequency low (that is, increasing the filtering effect), either the number of steps in the shift register may be increased or the cycle of the clock signal that sets the timing at which data is shifted may be prolonged.
However, in a circuit structure like that of the conventional example described above, when used with the digital encoder signal passed through the LPF, if the signal is put through the LPF after the digital encoder signal changes, until that digital encoder signal change is confirmed, a time delay occurs that corresponds to the number of steps in the LPF shift resister and the data shift timing.
That is, when the cut frequency is set low (the filtering effect is large), a large time delay occurs after the digital encoder signal changes and until that change is confirmed.
However, a problem arises in that this type of delay, for example in a case in which a digital encoder is used for the head drive control on a serial printer that prints by moving back and forth (that is, reciprocally) a printhead that discharges ink droplets, greatly increases the reciprocal registration adjustment for correcting the discharged position of the ink droplets during reciprocal printing.
Also, when performing control like that of a motor drive used for a serial printer, with its repeated stops, drives and reverses, that is, when there are large variations in velocity, when the digital encoder LPF cut frequency is low, that is, when the time from when the digital encoder signal changes until the time that change becomes confirmed is long, the difference between the physical position (the position at which the digital encoder signal changes) and the position determined by the encoder signal that has been passed through the LPF differs greatly between fast velocity and slow velocity. Accordingly, a great difference arises between the position recognized by the control circuit and the actual position of the carriage, which prevents precise positional control.